1. Field of the Invention
This invention relates to an automatic transmission, particularly to an automatic transmission for an automobile which is used together with a torque converter, and more particularly to an automatic transmission capable of performing lock-up clutch function.
2. Description of the Prior Art
In general, an automatic transmission mechanism comprises a plurality of planetary gear units, an element of which is connected with an input member by a clutch and locked by a brake or a one-way clutch to provide forward three speed or forward four speed transmission stages.
With a recently proposed automatic transmission, a lock-up clutch is arranged in a torque converter disposed between an engine output shaft and the input shaft of the above-mentioned automatic transmission mechanism, wherein the output shaft and the automatic transmission mechanism are mechanically coupled by the lock-up clutch in transmission stage higher than a certain transmission stage, particularly in the highest stage (overdrive), providing an elimination of power loss caused by the torque converter and an improvement of fuel efficency.
However, with the above-mentioned automatic transmission, since the lock-up clutch is incorporated in the torque converter, the dimension along the axis expands that much longer. In a recent growing tendency that small-sized structures are in demand for F.F (front-engine and front-drive) design or the like, said longer structure leads to problems such as limitation of flexibility associated with loading on vehicles, need of particular regulation for lock-up clutch and complicated structure of hydraulic control mechanism.
Further, since a lock-up clutch incorporated in a torque converter consists of a single plate clutch to be switched depending on oil flow direction, it has less torque transfer capacity and a lower reliability in comparison with a multi-plate clutch to be regulated by a hydrauric servomechanism comprising a cylinder means.